Harpenden: Petrol in a Diesel Car? How Worried to Be
If it has not been started, worry less than you likely are. Petrol resting in a diesel tank does nothing to your motor till something pulls it through your fuel system — your ignition priming your pump, the starter, or the drive itself. So the fair answer to the first question anyone in Harpenden asks, "have I ruined it?", is generally no. Keep it like that: leave the motor off then call Petrol in Diesel. What petrol actually does to a diesel, how to judge how far it's travelled, and how to keep it from happening a second time: that's the rest of this page.
What petrol in a diesel does to your engine
Passing through a motor, diesel is doing two things at once. It ignites, and before it combusts it protects. What keeps the surfaces inside the high-pressure pump and your injectors from wearing down against each other is the diesel flowing through them — they are built to very fine clearances.
The second task petrol does not do at all, and the first it does badly. It is thinner than diesel and strips away the protection the system hinges on, and so a pump running on a petrol mixture is a pump wearing itself. And petrol is made to resist igniting under compression, which is the reverse of what a diesel needs from its fuel. That is where the rattling comes from.
Together, those make the shape of the trouble clear. Not contact but circulation is what does the damage. Petrol in a tank is a fuel fault. After your pump has been working on that petrol, a mechanical fault has begun: whatever is shed by a pump that is wearing is carried onward to the injectors with the fuel.
How bad is petrol in a diesel? Four scenarios our technicians see around Harpenden
Filled but never started — the best it gets. The petrol is in your tank and just your tank, and the job is a drain and a flush where the vehicle stands. This is the one situation that can only truly take place on a forecourt, because anywhere else the car has been driven to get there.
A moment's running, or just your ignition on. With the system primed, the lines and filter now contain petrol that has come from the tank. Still generally simple: drain, flush, and the filter seen to.
Driven a few miles. Now your pump has spent these miles working on thinned fuel. The work is your tank, your lines, the filter and fresh fuel through the entire circuit, and how hard the car was driven matters as much as how far. The school run to St Nicholas CofE VA Primary School is one matter. Any distance on the M1, under 3 miles from Harpenden, means the motor working hard throughout, and a run to St Albans, under 5 miles away, is a very different job as well.
Driven till it protested. Knocking, a lumpy idle, smoke, a throttle that has gone soft. That is your motor telling you its fuel has stopped behaving like diesel. Leave it wherever it stops: a lay-by, a side street, the verge on Rothamsted Avenue. Long-term damage is very rare if a vehicle is pulled over early, and the drain and flush that still come first take care of it.
What Harpenden drivers get told about petrol in a diesel — and the myths in it
Myth: "Fill it the rest of the way with diesel and it will be fine." It will not. A fuller tank of the same mix is all you get, because topping up removes nothing. The assumption behind that guidance — that diesel fuel systems tolerate petrol by percentage — is wrong.
Myth: a newer diesel copes with a bit of petrol. Rather, the opposite. More precise components and higher pressure are the way a current diesel runs its fuel, and that makes the protection count more, not less.
This is the one that catches drivers. Myth: "If it starts and drives, there is no harm done." Diesel in a petrol motor usually means a motor that will not run: the motorist stops, and so does the damage. Nothing seems to have happened when a diesel fed petrol fires up, ticks over and drives away, as it usually will; meanwhile the pump wears. Being able to run is not proof of being fine.
"A drop of two-stroke oil and drive it home" — a myth, taken from a fix from years ago, for a different fault. It doesn't put back what petrol has thinned, and that petrol is still in that tank later on.
Myth: "Drain the tank yourself and you're sorted." After the engine has been run, some of that petrol is no longer inside that tank: it is in the fuel filter and the fuel lines, and it goes right back through your pump on the next start. And, a can of petrol filled by the kerbside is genuinely hazardous.
How does petrol in a diesel occur?
On purpose? Never. Through carelessness? Seldom, if ever. A diesel nozzle mostly will not go into a petrol vehicle, yet the smaller petrol nozzle goes into the filler of a diesel almost without resistance — so forecourts make this one mistake easy, and only in one direction.
It happens when habit and the vehicle stop matching. Courtesy car, hire car, or a vehicle that is brand new. A household with one petrol and one diesel on the drive. The pumps in a different order on a well-known forecourt. A hurried fill on the way to the nearest station with a train to catch. Precisely the moment it happens: after years of driving petrol, someone fills a diesel Volkswagen Tiguan for the first time. Your hand repeats what it has always done once you add rain, a phone call or a back seat full of children.
Harpenden drivers: how to prevent petrol in a diesel next time
- Not every forecourt in the UK keeps to green for unleaded and black for diesel, even if most usually do. Check the petrol pump's label; the colour is just a hint.
- The first fill in a new vehicle is the time to check your filler flap — inside it, or on the cap, many cars state the fuel to use.
- One of each in the household? Label the key to your diesel — a coloured fob, a sticker — because meaning to remember is not enough.
- It sounds daft and it does the trick: on the first fill in a new or hired car, say "diesel" aloud before you take the nozzle off the pump.
- Tuck the phone away. This is an easy mistake to make while you're thinking about something else.
- Up to a point, a misfuel inhibitor helps: some diesels come with a filler built to reject a petrol nozzle, and add-on devices exist for the remainder. Count it as a safety net, and keep checking the pump.
- Less petrol makes a thinner mix, so stop the moment you realise — though remembering that the next stage is unchanged. It is still not to crank it.
Petrol already in a diesel in Harpenden? Begin here
Short and simple, then, is the order of things. Don't crank it, and don't turn the ignition on. Still at the forecourt, at BP or anywhere else? Ask the staff to help you push the vehicle clear of the pump. Next, 0333 366 0103. We will ask what went in, roughly how much, and if it has been run.
Corrosion and deposits form the issue with AdBlue, where petrol brings lost protection. So if it was AdBlue, say so — and read its own Harpenden page.
Why one petrol in a diesel task in Harpenden costs more than another
The four situations above are four different tasks, so no single quote for this would be honest. Mostly, your cost moves with which one you are in. Two things come after that: the petrol that was put in as a share of what the tank holds, then the spot your car is in and whether our van can get to it safely.
Three queries come first, before any quote, rather than after — what got in, how much, has it run — and this is the reason. A range becomes a straight figure when those answers are in.
About Harpenden and its setting
Harpenden is a town in Hertfordshire, situated near the lakes and rivers of the Eastern region, with Harpenden Common and other green spaces nearby. Harpenden sits at an elevation of around 108 metres, close to the city of St Albans and approximately 23.5 miles from London. Harpenden has a mix of housing types, but is predominantly characterised by detached homes, with nearby areas including Batford and Hatching Green. Harpenden is home to around 18,031 people, according to recent estimates.
Across Hertfordshire and in Harpenden: fuel drain cover
Harpenden rests in the AL postcode area, and the teams that answer petrol in diesel jobs there work the places near it as well — Harpenden Common, Batford and Hatching Green among them. A forecourt, a supermarket car park, a lay-by or your own drive are all spots our engineers work, and your car doesn't need to come to us.
At the pump in Harpenden, still?
The best place there is, then — that is where you are. Keep the car turned off, then ring.